An image sensing apparatus using a solid-state image sensing device, such as a digital camera, now finds use in a variety of applications. There are instances where an image sensing device develops pixel failure, namely a phenomenon in which failure occurs pixel by pixel during the course of or after manufacture, resulting in output of signal having an abnormal level. The number of pixels used in solid-state image sensing devices is increasing year by year and it is conceivable that this will be accompanied also by an increase in the number of defective pixels.
In an image sensing device such as a CCD, if a phenomenon of local sensitivity failure developed in a semiconductor occurs during the course of or after manufacture among pixels arranged in two dimensions, output of charge corresponding to a quantity of incoming light can not be obtained. This causes a conspicuous white or black dot unrelated to an object in a sensed image as so-called failure pixel. As methods for correcting the white or black dot, a method of replacing the pixel value of a failure pixel with the pixel value of the adjoining pixel and a method of interpolating the pixel value of a failure pixel with a median or average of pixel values of the neighboring pixels have been proposed as described in Japanese Patent Application Laid-Open No. 10-42201, for instance.
Pixel failure involves a variety of characteristics. In particular, with regard to defective pixels having a characteristic such that the high level signal appears on all vertical lines of the image sensing device, the display error that occurs on the display screen of the image sensing device owing to such failure is readily noticeable by the human eye since the failure appears as a white line (called white defect). Moreover, owing to the trouble that ensues when there is a change in the level of the defect due to temperature or aging, various correction schemes have been proposed heretofore and display error has become inconspicuous owing to such correction. For example, the specification of Japanese Patent Application Laid-Open No. 07-067038 describes a correction method comprising averaging and deleting random noise components by integrating (adding and averaging), on a per-pixel basis, an output signal of an optical black pixel in a vertical direction of a solid-state image sensing device, detecting a fixed-pattern noise signal by line, and subtracting the fixed-pattern noise from the output signal of an effective pixel area of the solid-state image sensing device by line, whereby it is possible to improve upon dark current components, white lines and smear, etc., which are fixed-pattern noise.
However, with the prior-art correction scheme of the kind described in the specification of Japanese Patent Application Laid-Open No. 10-42201, there are instances where the image after correction becomes discontinuous in a case where a boundary between light and dark regions is present in the vicinity of a defective pixel or in a case where there is a sudden change in contrast in the vicinity of a defective pixel.
Further, with the prior-art correction scheme of the kind described in the specification of Japanese Patent Application Laid-Open No. 07-067038, the result obtained by adding and averaging the output signal of the optical black pixel area is subtracted uniformly from the output signal of the effective pixel area. As a result, though black-current noise and white defects are corrected, there is a possibility that noise will increase due to an effect of random noise, for instance, present in an optical black image, where defects are absent. Further, an overcorrection in which the signal is subtracted excessively occurs in a case where the output signal from an optical black pixel area or effective pixel area saturates or in a case where, in the correction of a defective pixel of a white defect having a characteristic in which the noise appears on all vertical lines of the image sensing device, the output of the vertical-line signal produced from the defective pixel is non-uniform owing to the position of the defective pixel. A problem which arises is that there is a needless decline in image quality.